Method for operating an ink jet printer

ABSTRACT

A method of operating for an ink jet printer having a printer head bearing a plurality of equi-distantly spaced ink jets for printing parallel lines of dots on a substrate in n passes of the print head over the substrate. Between each pass of the printer head over the substrate, the printer head is displaced in a direction perpendicular to the direction of the lines by a distance y that satisfies: (i) y=m 1 ×a where m 1  is a natural number equal to 2 or more and a is the separation gap between adjacent lines; (ii) y is not a multiple of the inter ink jet separation distance of the printer head; and (iii) m 1  and n do not have a common factor except the factor with the value of 1. These conditions on y ensure that two adjacent lines are printed by different ink jets and that each line is printed by only one ink jet.

FIELD OF THE INVENTION

The present invention relates to a method of operation for directcomputer-to-print printing systems in general and ink jet printers inparticular.

BACKGROUND OF THE INVENTION

In EP 0 023 433, there is illustrated and described a high speed ink jetprinter of the rotatable drum type having a print head for printing animage on a substrate during n printing passes where b=n×a, “b” is theink jet head separation gap between adjacent ink jet heads and “a” isthe inter ink dot line separation gap between adjacent ink dot linesi.e. a pixel length, each printing pass being constituted by a drumrevolution and the displacement of the print head along the substratethrough a step equivalent to the pixel length between consecutive drumrevolutions.

FIG. 1 shows an ellipse shaped image printed during three drum rotationsof such an ink jet printer having 5 ink jet heads labelled A, B, C, Dand E. For convenience, the ink dot lines printed by the different inkjet heads during each consecutive drum rotation are denoted by thereference numbers 1, 2 and 3. It can be readily appreciated that theimage suffers from the drawback that a single defective ink jet head Chaving, say, a blocked nozzle, causes an unprinted band of a width equalto the inter ink jet head separation gap.

It is the object of the present invention to substantially overcome thisand other disadvantages.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, there isprovided a method of operation for a high speed ink jet printer having aprint head for printing an image on a substrate, the print head beingsubstantially coextensive with the substrate and displaceable relativethereto in substantially mutually orthogonal first and seconddirections, the method comprising the steps of:

(a) displacing the print head in the first direction between initial andfinal printing positions for printing ink dot lines on the substrate ina manner determined by the appearance of the image to be printed;

(b) displacing the print head in the second direction through a step “y”between consecutive initial printing positions of the print head; and

(c) repeating steps (a) and (b) during n printing passes where b=n×a,“b” is the ink jet head separation gap between adjacent ink jet headsand “a” is the inter ink dot line separation gap between adjacent inkdot lines at the predetermined line resolution;

characterized in that the step “y” satisfies the following conditions:

(i) y=m₁×a where m₁ is a natural number equal to 2 or more,

(ii) y≠m₂×b where m₂ is a natural number equal to 1 or more,

(iii) b=m₃×a where m₃ is a natural number equal to 2 or more and

(iv) m₁ and m₃ do not have a common denominator.

In accordance with the teachings of the present invention, thedisplacement of the print head relative to the substrate through a stepgreater than the inter ink dot line separation gap entails that adjacentink dot lines are printed by different ink jet heads. The main advantageprovided for by this method of operation is that in the case of aninoperative ink jet head, the ink dot lines which it should have printedare spaced out along the image rather than being adjacent to one anotherin an unprinted band as hitherto occurs with a conventional ink jetprinter as described hereinabove such that their detrimental effect onimage quality is considerably less noticeable.

This method of operation requires a longer print head for printing thesame size image as a conventional ink jet printer. This is because, atany one time, one or more ink jet heads are positioned in inoperativeprinting positions either to the left or right of a substrate. Thus,during the printing of an image, some or all of the ink jet heads ininitially inoperative printing positions are displaced to operativeprinting positions whilst some or all of the ink jet heads in initiallyoperative printing positions are displaced to inoperative printingpositions. Some conventional ink jet printers can be retro-fitted so asto operate in the above described manner for printing narrower thanusual images.

The spacing between consecutive ink dot lines printed by a particularink jet head, i.e. the length of the step “y” by which a print head isdisplaced relative to a substrate in the second direction, is notarbitrary but rather must comply with a number of constraints as nowdescribed assuming that all parameters are in the same units. The firstconstraint to be complied with is that the spacing between consecutiveink dot lines of a particular ink jet head is equal to a multiple of theseparation gap “a” between adjacent ink dot lines at a desired lineresolution, namely, y=m₁×a where m₁ is a natural number greater thanone. The second constraint to be complied with is that the spacingbetween consecutive ink dot lines of a particular ink jet head is notequal to a multiple of the inter ink jet head separation gap “b”,namely, y≠m₂×b where m₂ is a natural number greater than zero, such thatink jet heads do not assume previously occupied positions relative to asubstrate. The third constraint to be complied with and which alsoapplies to conventional ink jet printers is that the inter ink jet headseparation gap “b” is a multiple of the inter ink dot line separationgap “a” at a particular desired line resolution, namely, b=n×a where nis a natural number greater than zero. However, in addition, in view ofthe first constraint y=m₁×a, also m₁>n and m₁ and n do not have a commonfactor such that ink jet heads do not assume previously occupiedpositions.

In line with conventional practice, an ink jet printer operative inaccordance with the teachings of the present invention can preferablyprint at one or more preset line resolutions, for example, a low lineresolution of, say, 40 lpi or a high line resolution of, say, 60 lpi,the trade-off for a higher resolution, of course, being in terms of aslower throughput. As is known in the art, for all such line resolutionsettings, the relationship b=n×a applies but for different values of n.

A further feature of an ink jet printer operative in accordance with theteachings of the present invention is that the unprinted ink dot linescaused by a defective ink jet head can be at least partially compensatedfor by suitable manipulation of partially printed ink dot lines in theirvicinity so as to restore picture quality as perceived by an observereven when an ink jet printer has one or more defective ink jet heads.The compensation of an unprinted ink dot line can be achieved byprinting a printed ink dot line adjacent to an unprinted ink dot linewith either additional ink dots or larger ink dots, if possible, so asto restore the original dot percentage. In color ink jet printers, theabove described compensation technique is applied for each primary colorseparately.

The detection of a defective ink jet head can be achieved by an operatorduring, say, a routine start-of-day work procedure. Alternatively, itcan be achieved automatically by means of a dedicated ink jet head testmodule including image processing and pattern recognition capabilitiesfor processing the image of a printed image as provided by a cameradirected theretoward.

Also in line with conventional practice, an ink jet printer operative inaccordance with the teachings of the present invention preferably printsan image as a matrix of cells, each cell in turn being printed as amatrix of pixels. As known in the art, a separation gap “c” betweenadjacent cells alone the direction in which the print head isintermittently displaced fulfills the following two relationships:c=m₄×a and b=m₅×c where m₄ and m₅ are natural numbers greater than zero.The spacing between consecutive ink dot lines of a particular ink jethead is preferably greater than the separation gap “c” such that aparticular ink jet head prints a maximum of one ink dot line in any onecell. In this case, the compensation of an unprinted ink dot line ispreferably performed at the cell level in the sense that ink dot lineswithin a cell containing an unprinted ink dot line are printed witheither additional ink dots or larger ink dots, if possible, so as torestore the original dot percentage. In the case that an unprinted inkdot line occurs at the edge of a cell, its dot percentage may berestored by modifying the adjacent printed ink dot line in itsneighboring cell.

Ink jet printers in accordance with the teachings of the presentinvention can either be of the rotatable drum type or the flat bed type.In addition, they can be of the “continuous” ink jet printer type inwhich each ink jet head provides a continuous flow of ink drops of whichsome impinge on a substrate at desired printing locations whilst othersare disposed of in a manner determined by the appearance of an image tobe printed or the “impulse” or “drop-on-demand” ink jet printer type inwhich each ink jet head is independently actuated to expel ink drops ina manner determined by the appearance of an image to be printed.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame can be carried out in practice, reference will now be made, by wayof a non-limiting example only, to the accompanying drawings in which:

FIG. 1 illustrates an ellipse shaped image printed by a conventional inkjet printer having a defective ink jet head in the form of a blockednozzle;

FIG. 2 is a schematic view of a rotatable drum type ink jet printerconstructed and operative in accordance with the teachings of thepresent invention;

FIG. 3 is a front view of the print head of the ink jet printer of FIG.2 including four arrays of ink jet heads for four color processprinting;

FIGS. 4A and 4B are a flow chart illustrating the method of operationfor the ink jet printer of FIG. 2, FIG. 4B being a continuation of FIG.4A;

FIG. 5 illustrates the same ellipse shaped image printed by the ink jetprinter FIG. 2 having a defective ink jet head in the form of a blockednozzle;

FIGS. 6A-6C illustrate the operation of the ink jet printer of FIG. 2for correcting the image of FIG. 5; and

FIG. 7 is a schematic view of a flat bed type ink jet printerconstructed and operative in accordance with the teachings of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 2 shows an ink jet printer, generally designated 1, of the“drop-on-demand” type including a foraminous drum 2 bearing a removablesubstrate 3 preferably secured thereto by application of a vacuum to itsunderside. The drum 2 is rotatable about a shaft 4 under the control ofa drum controller 5 constituting part of an ink jet printer controller 6for controlling the operation of the ink jet printer 1. Each substrate 3is loaded onto the drum 2 and unloaded therefrom in a conventionalmanner be it manual or automatic.

A print head carriage 8 is mounted parallel to the shaft 4 and isdisplaceable therealong under the control of a print head carriagecontroller 9. The print head carriage 8 includes a print head 10 whichextends along the near entire length of the drum 2 corresponding to themaximum width of substrate 3 to be printed thereon. As shown in FIG. 3,the print head 10 includes four ink jet head arrays 11, 12, 13 and 15for printing the four primary CMYK colors, namely, cyan, magenta, yellowand black. Each ink jet head array 11, 12, 13 and 15 is supplied withink from a dedicated ink reservoir 11′, 12′, 13′and 15′, respectively.The ink jet heads of the different arrays are not in registration withone another so as to ensure that they do not superimpose differentlycolored ink dot lines which would tend to mix, thereby lowering printquality.

Individual ink jet heads are actuated by an ink jet head driver 16driven by a pixel pattern generator 17 which determines which pixels ofa particular cell are to be filled by dots and the dot area of each ofthe selected dots in accordance with the dot percentage values of eachcolor in that cell. The pixel pattern generator 17 includes a library ofpatterns for generating different dot percentages. In some cases, thelibrary only contains a single pattern to achieve a particular dotpercentage, for example, 0% dot percentage or 100% dot percentage.However, in practice, each cell is represented by a m×n pixel matrix,say, 6×6, such that there are a large number of possible patterns toachieve a particular dot percentage of, say, 40% dot percentage. Forexample, in a simplified case, there are four patterns which achieve a25% dot percentage in a 2 by 2 cell matrix, namely, a dot in one cornerof the cell matrix. Input to the pixel pattern generator 17 is providedfrom an external data source 18, for example, removable media i.e. anoptical disk, or a desk-top publishing (DTP) system.

The operation of the ink jet heads of the print head 10 is monitored byan ink jet head test module 19 including image processing and patternrecognition capabilities for processing the image of a printed image asprovided by on-line or off-line image acquisition apparatus, say, acamera 20, directed theretoward. In the case that one or more ink jetheads are found to be defective, typically, due to a blocked nozzle, theink jet head test module 19 updates the pixel pattern generator 17accordingly such that certain patterns which would require the actuationof defective ink jet heads are temporarily degenerated.

The principles of operation of the ink jet printer 1 are now describedwith reference to FIGS. 4A and 4B. First, an operator downloads a filecontaining a pixelized image to be printed from a suitable source, forexample, an optical disk. The operator then selects the desired lineresolution at which the image is to be printed from the available rangeof line resolutions of the ink jet printer 1. Finally, the operatorselects the desired spacing between consecutive ink dot lines of an inkjet head from the available range of spacings which, in turn, dictatesthe length of the step “y” through which the print head carriage 8 isdisplaced between consecutive rotations of the drum 2.

Based on his selections, the ink jet printer controller 6 determines thenumber of steps n through which the print head carriage 8 is required tobe displaced so as to print an image of a predetermined width at thepredetermined line resolution. Thereafter, the pixel pattern generator17 selects a pattern of dots for each color of each cell according toits dot percentage from its library. The pixel pattern generator 17provides the patterns to the ink jet head driver 16 which actuates therequired ink jet heads in accordance with the appearance of the image tobe printed during successive rotations of the drum 2 whist advancing theprint head carriage 8 through the step y between consecutive drumrotations.

An example of the operation of the ink jet printer 1 is shown in FIG. 5for printing the same ellipse shaped image as shown in FIG. 1 during thesame three drum rotations. As before the ink jet lines printed duringeach drum rotation are designated by the reference numbers 1, 2 and 3for the first, second and third drum rotations, respectively. The printhead 10 includes 7 ink jet heads labelled P, Q, R, S, T, U and V ofwhich the ink jet heads P and Q are located in initially inoperativeprinting positions and ink jet heads R, S, T, U and V are located ininitially operative printing positions in the initial home position ofthe print head carriage 8.

Thus, in this case, for a step “y” equivalent to the separation gapbetween four ink dot lines, printing, of the ellipse shaped image isachieved by actuating the ink jet heads R, S, T, U and V in the initialhome position of the print head carriage 8 during the first drumrotation, actuating the ink jet heads Q, R, S. T and U during, thesecond drum rotation and actuating the ink jet heads P. Q, R, S and Tduring the third and final drum rotation. It can therefore be readilyseen that whilst the ink jet heads P and Q are displaced frominoperative printing positions to operative printing positions duringthe advancement of the print head carriage 8, the ink jet heads U and Vare displaced from operative printing positions to inoperative printingpositions.

In the case that the ink jet head S is defective with a blocked nozzle,it will be readily noticed that the unprinted ink dot lines shown dashedand labelled S1, S2 and S3 caused thereby are spaced apart and thereforeless conspicuous to an observer than in the case that they were adjacentto one another as hitherto occurred as described in the Background ofthe Invention as can be best appreciated by comparing the ellipse shapedimage shown in FIG. 5 to its counterpart in FIG. 1.

The image quality of the ellipse shaped image shown in FIG. 5 can befurther improved by the ink jet head test module 19 as now explained.For example, assuming that all the ink jet heads P, Q, R and S areoperative, FIG. 6A shows a 4 by 4 pixel cell having, say, a 50% dotpercentage value achieved by each of the ink dot lines printed by theirrespective ink jet heads containing two ink dots as determined by thepixel pattern generator 17. However, as evidenced in FIG. 6B, the samecell would only have a slightly less than, say, 30% dot percentage valuein the case that the ink jet head S is inoperative. In such an event,the ink jet head test module 19 would update the pixel pattern generator17 as to the status of the ink jet head S such that the pixel patterngenerator 17 can instruct the ink jet head driver 16 to actuate theoperative ink jet heads P, R and Q so as to print the cell with thedesired 50% dot percentage value, for example, as shown in FIG. 6C.

FIG. 7 shows a flat bed type ink jet printer 23 having a substratebearing flat bed 24 which performs a reciprocating motion in a firstdirection and a print head carriage 25 carrying a print head 26 with inkjet heads for printing parallel ink dot lines on a substrate 27 during aforward stroke of the flat bed 24. Thus, in this case, printing of animage is achieved by repeatedly returning the flat bed 24 to its homeposition by a return stroke, advancing the print head carriage 25 in atraverse direction relative to the flat bed 24 by a step “y” asdescribed hereinabove and actuating the ink jet heads at each newposition whilst displacing the flat bed 24 through a forward stroke soas to build up the image over a number of forward strokes of the flatbed 24. For the economy of time, it will be readily appreciated that thereturn stroke of the flat bed 24 and the step advancement of the printhead carriage 25 can be performed simultaneously.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention can be made bythose ordinarily skilled in the art.

What is claimed is:
 1. A method of operation for an ink jet printerhaving a print head defining a printing width between the first and lastink jet heads for printing an image on a substrate at a predeterminedline resolution, the print head extending in a second direction alongthe near entire width of substrate bearing means and being displaceablerelative to the substrate in substantially mutually orthogonal first andsaid second directions, the method comprising the steps of: (a)displacing the print head in the first direction between initial andfinal printing positions for printing ink dot lines along the entirelength of the substrate in said first direction as a function of theappearance of the image to be printed; (b) displacing the print head inthe second direction through a step y between consecutive initialprinting positions of the print head; and (c) repeating steps (a) and(b) during n printing passes where b=n×a, b is the ink jet headseparation gap between adjacent ink jet heads and a is the inter ink dotline separation gap between adjacent ink dot lines at the predeterminedline resolution where n is a natural number equal to 1 or more;characterized in that the printing width of the print head is wider thanthe image to be printed and the step y satisfies the followingconditions: (i) y=m₁×a where m₁ is a natural number equal to 2 or more,(ii) y≠m₂×b where m₂ is a natural number equal to 1 or more, (iii) m₁and n do not have a common factor except the factor with the value of 1.2. The method according to claim 1 wherein the printed image isconstituted by a matrix of cells where c is the separation gap betweenadjacent cells constituting the printed image, wherein: c=m₄×a; m₄ is anatural number equal to 2 or more; and y>c.
 3. The method according toclaim 2 further comprising the steps of: (d) off-line detecting areduced gray level within a portion of the printed image constituted byone or more cells; and (e) adjusting a dot percentage of one or morecells constituting the portion so as to substantially restore the graylevel to a predetermined value.
 4. The method according to claim 1wherein y>b.
 5. The method according to claim 1 wherein the ink jetprinter is operative in a drop-on-demand mode of operation.
 6. Themethod according to claim 1 wherein the ink jet printer is operative ina continuous mode of operation.
 7. The method according to claim 1wherein the method is operative for a rotatable drum type ink jetprinter.
 8. The method according to claim 1 wherein the method isoperative for a flat bed type ink jet printer.